Unsaturated ester resins have been widely used in various applications such as coating materials and molding compositions containing reinforcements like glass fibers. Meanwhile, cured unsaturated ester resins (hereinafter referred to as “unsaturated ester cured products”) are inherently so brittle that toughening is required for practical use.
Heretofore, various techniques have been disclosed to toughen unsaturated ester cured products. For example, there have been disclosed methods of adding flexibilizers such as a conjugated diene polymer (Patent Document 1), a carboxyl-terminated reactive liquid polymer (Patent Document 2), and a vinyl-terminated reactive liquid polymer (Patent Document 3) to an unsaturated ester resin. Although the toughness is improved in the cured products obtained by these methods, they are still inadequate and sometimes deteriorated in other properties such as mechanical properties (such as elastic modulus).
Patent Document 4 discloses a method of adding an epoxy resin to an unsaturated ester resin to prepare a resin composition and then curing the composition using a radical initiator. In this method, however, there are cases where an uncured epoxy resin emerges to the surface of the cured product to deteriorate the surface state (the surface becomes tacky) or the toughness of the cured product cannot be sufficiently improved.
Patent Document 5 discloses a method of adding a radical initiator and an epoxy resin curing agent to a resin composition containing an unsaturated ester resin and an epoxy resin in combination. This method, however, cannot improve the toughness of the unsaturated ester resin notably. In addition, there are also cases where a curing reaction of the epoxy resin is insufficient due to a reaction between the epoxy resin curing agent and a double bond of the unsaturated ester resin, resulting in no improvement in the surface state.
Recent studies (for example, Non-Patent Document 1) report that addition of an acrylonitrile-butadiene-styrene (ABS) resin to an unsaturated ester resin improves the toughness of a cured product to be obtained. However, the method sometimes has to sacrifice the elastic modulus, which is one of the most important properties of cured products.